UCNtau: A Precision Measurement of the Neutron Beta-Decay Lifetime*

Eighty years after Chadwick discovered the neutron, physicists today still debate over how long the neutron lives. Measurements of the neutron lifetime have achieved the 0.1\% level of precision ($\sim$ 1 s), however, experiments using the bottle technique yield lifetime results systematically lower than those using the beam technique. Measuring the neutron lifetime is difficult due to several limitations: the low energy of the decay products, the inability to track slow neutrons, and the fact that the neutron lifetime is long ($880.3 \pm 1.1$ s, PDG2014). In particular, slow neutrons are susceptible to many loss mechanisms other than beta-decay, such as upscattering and absorption on material surfaces; they act on time scales comparable to the neutron beta-decay and thus make the extraction of the beta-decay lifetime very challenging. In the UCN$\tau$ experiment, we trap ultracold neutrons (UCN) in a magnetic-gravitational trap. The apparatus, installed at the Los Alamos UCN source, has been used to develop new techniques--using field confinements with attentions to the phase space evolution of trapped neutrons--with an aim to reduce the uncertainty to 1 s (and better). I will report first competitive results and discuss plans to quantify systematic effects.